An upside down frown would be a smile. This is largely a stand-alone phrase or whole sentence (complete thought) and can be used as you have typed it. It is a phrase most commonly used with children to get them to smile when they are sad or angry over relatively minor issues but not commonly used with adults or for serious problems. it is generally considered to be inappropriate for serious problems where deeper conversation or consolation would be expected or in formal or professional relationships.

It means to smile. Often parents say this to little children who are sulking. Example: Come on cheer up. Turn your frown upside down and let's have fun.